The present invention relates to a method for preparing a room temperature vulcanizable polyisobutylene polymer. More particularly, the invention relates to a method for synthesizing a polyisobutylene having hydrolyzable silyl-functional groups positioned at or near molecular chain ends, said polymer being prepared by a living polymerization wherein isobutylene, a silyl-functional cationic initiator and a silyl-functional comonomer are reacted in the presence of a Lewis acid.
Living anionic polymers are well known in the art but truly living cationic systems have only been developed more recently. By definition, termination and chain transfer rates are zero, or negligible, in such a living polymer so that the concentration of active centers remains approximately constant. Of particular commercial interest are living organic polymers which can be used to prepare telechelic oligomers and macromolecules having hydrolyzable silyl end groups since these can be used to formulate room temperature vulcanizable (RTV), moisture-curable sealants, caulks and adhesives, inter alia. Unlike the well known moisture-curable silicone RTVs, compositions based on polymers having a hydrocarbon backbone offer the advantage of low permeability, making them more suitable for gas barrier applications such as form-in-place gaskets, O-rings, rubber plugs/seals, and the like.
U.S. Pat. No. 4,829,130 to Licchelli et al. discloses a method for preparing silylated copolymers of isobutylene which are cross linkable upon exposure to moisture. In this case, isobutylene is copolymerized with a comonomer having a vinylic group attached to a silylalkyl group through a benzene ring connecting group. The product of this copolymerization is a polyisobutylene (PIB) having pendant silyl-functional groups along its main chain.
Likewise, U.S. Pat. No. 4,524,187 to Greco et al. teaches cross linkable polyisobutylene copolymers having pendant silyl functionality which is reactive with moisture. In this case, isobutylene is first reacted with a triene compound to form a copolymer having pendant vinyl groups, the latter then being silylated with a moisture-reactive hydridosilane or mercapto-functional silane.
Polymers having reactive silyl head groups were described by Kennedy et al. in Advances in Polymer Science, 43, 1-50 (1982). These workers discovered that initiators which included an SiCl or SiH group as well as a cationogenic moiety could be used to polymerize cationic systems such that the silicon-functional groups survived. The cationogenic moiety studied was the benzyl chloride group (i.e., xe2x80x94C6H4xe2x80x94CH2Cl) wherein xcex1-methylstyrene was polymerized in the presence of diethyl aluminum chloride as coinitiator. In this case, however, it was observed that the amount of xe2x80x94SiCl detected was only a small fraction of the theoretical, especially when more than one chlorine was present in the head group. Further, although these workers suggest that the SiCl groups may survive polymerization of isobutylene, they did not obtain quantitative confirmation thereof and did not demonstrate the new initiators in connection with this monomer. Thus, Kennedy et al. then turned to the investigation of systems having SiH terminal groups, such polymers being considered more promising intermediates which presented fewer experimental difficulties.
In copending application Ser. No. 292,333 filed on Apr. 15, 1999, we disclose the use of a novel class of compounds which, unlike the initiators taught by Kennedy et al., cited supra, can initiate cationic polymerization in a controlled manner to provide a living polymer having a silicon halide head group. This living polymer is prepared by reacting at least one cationically polymerizable monomer with the novel initiator, further described infra, in the presence of a Lewis acid. According to the above disclosure, the living cationic polymer may be further reacted with a coupling agent to provide a silyl-functional telechelic polymer. However, this method requires an exact stoichiometric amount of the coupling agent with respect to the living polymer chain, this often being difficult in large scale operations. Further, this method is relatively complicated and there is still a need for simpler means of synthesizing silyl-functional telechelic hydrocarbon polymers which can be readily cured by exposure to moisture.
It has now been discovered that a virtually telechelic, silyl-functional polyisobutylene can be prepared by reacting isobutylene, the above mentioned novel initiator, and a silyl-functional styryl comonomer in the presence of a Lewis acid.
The present invention, therefore, relates to a method comprising reacting, in the presence of a Lewis acid,
(A) isobutylene;
(B) an initiator of the formula
and; 
(C) a styryl comonomer of the formula 
wherein R is selected from H or methyl group, Rxe2x80x2 is a divalent non-aromatic hydrocarbon group having 1 to 6 carbon atoms, Rxe2x80x3 is independently selected from alkyl groups having 1 to 10 carbon atoms or aryl groups having 6 to 10 carbon atoms, Rxe2x80x2xe2x80x3 is a divalent non-aromatic hydrocarbon group having 2 to 6 carbon atoms, X is independently a halogen group, Y is halogen and n is independently 1, 2 or 3.
The invention further relates to the virtually telechelic silyl-functional polymer prepared by the above method.
Component (A) of the present invention is a monomer consisting essentially of isobutylene and may be either a substantially pure isobutylene (IB) monomer or a mixture of isobutylene with up to 20 mole percent of at least one monomer selected from 1-butene, 2-butene, isoprene, styrene, xcex1-methylstyrene or p-methylstyrene. These mixtures having a minor proportion of the above mentioned monomers are also referred to as xe2x80x9cisobutylenexe2x80x9d for the purposes herein. Preferably, component (A) is substantially pure (i.e., distilled) IB.
The reactive silyl-functional initiator (B) of the present invention is represented by the formula 
wherein R is selected from H or methyl group, Rxe2x80x2 is a divalent non-aromatic hydrocarbon group having 1 to 6 carbon atoms, Rxe2x80x3 is selected from alkyl groups having 1 to 10 carbon atoms or aryl groups having 6 to 10 carbon atoms, X is halogen, Y is halogen and n is 1, 2 or 3. Specific examples of Rxe2x80x3 include methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl, n-octyl, 2-ethylhexyl, n-nonyl, n-decyl, phenyl and tolyl, methyl being preferred. The group Rxe2x80x2 is exemplified by xe2x80x94CH2xe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94CH2CH2CH2xe2x80x94, xe2x80x94CH2xe2x80x94C(Me)2xe2x80x94 and xe2x80x94CH2xe2x95x90CH2xe2x80x94, and is preferably xe2x80x94CH2CH(Me)xe2x80x94, wherein Me hereinafter represents a methyl group. Specific examples of X and Y are fluorine, chlorine, bromine and iodine. Further, the moieties attached to the benzene ring of formula (i) may be in ortho, meta or para positions with respect to one another, the latter two isomers being preferred. Preferably, X and Y are chlorine and n is 2 or 3.
In a highly preferred embodiment of the instant method, the initiator has the formula 
wherein n is 2 or 3.
The initiators of the present invention can be prepared by first carrying out a hydrosilation reaction between a dialkenyl benzene species (or an alkenyl-alkynyl benzene species) and a halosilane, as illustrated below for a preferred system wherein Rxe2x80x2 is xe2x80x94CH2CH(Me)xe2x80x94: 
in which R, Rxe2x80x3, X and n are as defined above in connection with formula (i). In the above hydrosilation reaction, the molar ratio of component (iii) to component (iv) can be 0.3:1 to 10:1 but is preferably more than 1:1. More preferably, the ratio is 1:1 to 3:1. After the hydrosilation reaction, isolation of the desired product (v) by a suitable method, such as fractional distillation, is recommended. In this case, it is highly preferred that a radical scavenger, such as phenothiazine, be added to the mixture to prevent polymerization of the product and starting materials.
Suitable catalysts for the hydrosilation reaction are platinum black, platinum metal on various solid supports (e.g., carbon), chloroplatinic acid, alcohol solutions of chloroplatinic acid, and complexes of chloroplatinic acid with liquid ethylenically unsaturated compounds such as olefins and organopolysiloxanes containing ethylenically unsaturated hydrocarbon radicals bonded to silicon. Preferred catalysts include a platinum complex catalyst produced by reacting chloroplatinic acid and divinyltetramethyldisiloxane, prepared according to U.S. Pat. No. 3,419,593 to Willing and a neutralized complex of platinous chloride and divinyltetramethyldisiloxane, prepared according to U.S. Pat. No. 5,175,325 to Brown et al., these patents being hereby incorporated by reference. Most preferably, this catalyst is a neutralized complex of platinous chloride and divinyltetramethyldisiloxane. For this synthesis, the hydrosilation catalyst is generally employed at a level equivalent to a platinum concentration of from 0.1 to 1,000 parts by weight of platinum metal, preferably from to 1 to 100 parts by weight of platinum metal, per million weight parts (ppm) of the unsaturated component (iii).
After the hydrosilation reaction is carried out, synthesis of the initiator according to the present invention may be completed by hydrohalogenating structure (v). This reaction is again illustrated for the case in which Rxe2x80x2 is xe2x80x94CH2CH(Me)xe2x80x94 and Y is Cl: 
wherein R, Rxe2x80x3, X and n have their previously defined meanings. In the above hydrohalogenation reaction, the acid is typically added in excess.
Alternatively, the initiator may be prepared by a conventional Grignard synthesis (e.g., dicumyl chloride can be reacted with Mg followed by reaction with MeSiCl3).
The styryl comonomer (C) has the formula 
wherein Rxe2x80x3, X and n are as defined above for component (B) and wherein each may be the same or different from the corresponding selection in (B). In formula (vii), Rxe2x80x2xe2x80x3 is a divalent, non-aromatic hydrocarbon group having 2 to 6 carbon atoms. Again, the moieties attached to the benzene ring of formula (vii) may be in ortho, meta or para positions with respect to one another, the latter two isomers being preferred. It is preferred that Rxe2x80x3 is selected from methyl, ethyl or phenyl, X is chlorine, and n is 2 or 3. A particularly preferred styryl comonomer has the formula 
wherein n is 2 or 3.
The styryl comonomer (C) is known in the art and may be prepared by a hydrosilation reaction between a vinylalkenyl benzene and the corresponding halosilane in the presence of a platinum catalyst, followed by separation by distillation.
The Lewis acid, employed as a co-initiator in the present method, is of the type known to catalyze cationic living polymerization. Examples of such Lewis acids include BZ3, TiZ4, AIZ3, SnZ4 and ZnZ2, inter alia, wherein Z is independently selected from the group consisting of halogen, alkoxy having 1 to 4 carbon atoms and alkyl having 1 to 4 carbon atoms. Preferably Z is halogen, most preferably chlorine. Of course, those skilled in the art will recognize that a compound such as TiZ4 in which all of the Z groups are alkyl groups is not a useful catalyst for cationic living polymerization and such compounds are not contemplated herein. The interested reader is referred to the article by Matyjaszewski and Pugh in Cationic Polymerization Mechanism, Synthesis and Applications, Matyjaszewski, Ed.; Marcel Dekker, New York (1996), for a review of suitable Lewis acids. Preferably, the Lewis acid is selected from TiCl4, BCl3, Me2AlCl and Me2BCl.
In order to prepare a virtually telechelic, silyl-functional polyisobutylene of the invention, components (A) through (C) are reacted in the presence of the Lewis acid, the reaction being carried out either in a sequential manner or in a random manner. For the purposes of the present invention, the term xe2x80x9cvirtually telechelicxe2x80x9d denotes a polymer wherein the functional silyl groups do not necessarily reside at both terminal positions, as would be the case for a truly telechelic system. Rather, a functional silyl group derived from initiator (B) is located at a first terminal position (head) while, on average, a functional silyl group derived from comonomer (C) resides at or near the second terminal position (tail). Thus, the functional silyl group derived from comonomer (C) may be truly terminal or it may be, e.g., in a penultimate position or in a third, fourth or fifth position from the second terminus. Further, two or more of the latter functional units may congregate near the second terminus, this being controlled by the amount of component (C) used, as determined by routine experimentation.
In a first embodiment (sequential copolymerization mode), a monomer consisting essentially of isobutylene is polymerized with initiator (B) in the presence of a Lewis acid under a dry, inert atmosphere to provide a living cationic system. A Lewis base, such as pyridine or its derivatives, such as 2,6-ditert-butyl-pyridine (DTBP) or lutidine, is preferably included in this polymerization reaction in order to prevent initiation by protic impurities. Such cationic living polymerizations are well known in the art, including typical reaction conditions and proportions of the monomer, initiator, Lewis acid, etc. This polymerization is preferably run in a dry organic solvent solution (e.g., hexane, methylene chloride, methyl chloride, toluene, and combinations thereof). Of course, these conditions will vary according to the particular ingredients under consideration and doptimization of conditions and proportions needed to obtain a particular living polymer having a desired molecular weight can be achieved through routine experimentation by those skilled in the art. For example, the interested reader is directed to the chapter by Sawamoto in Cationic Polymerization Mechanism, Synthesis and Applications, Matyjaszewski, Ed.; Marcel Dekker, New York (1996) and the text by Kennedy and Ivan entitled Designed Polymers by Carbocationic Macromolecular Engineering: Theory and Practice published by Hanser (1992).
When the above described polymerization procedure is deemed complete, preferably when monomer conversion is at least 98%, the resulting living polymer is further reacted with styryl comonomer (C). Component (C) is preferably employed at a level of 1 to 20 times the molar concentration of initiator (B), more preferably 3 to 8 times the latter concentration. This reaction is preferably carried out at similar temperatures and in a similar solvent environment as described in connection with the preparation of the living polymer. The product of this sequential copolymerization is a virtually telechelic polymer having a halosilyl-containing group of the formula 
derived from initiator (B) at one terminus (head) and a halosilyl-containing group of the formula 
at, or near, the other terminal position (tail), wherein R, Rxe2x80x2, Rxe2x80x3, Rxe2x80x2xe2x80x3, X and n are as defined above.
In a preferred version of the first embodiment, a monomer of substantially pure isobutylene is reacted with an initiator according to formula (ii) in the presence of TiCI4. This reaction is carried out in a solvent, preferably a combination of methyl chloride and hexanes in a volume ratio of about 60/40 to 40/60, as well as in the presence of a Lewis base, such as DTBP, under dry conditions and at a temperature of xe2x88x92100xc2x0 C. to 0xc2x0 C., preferably xe2x88x9280xc2x0 C. to xe2x88x92600xc2x0 C. Preferably, the order of addition is solvent, Lewis base, Lewis acid, initiator and monomer. The concentrations of the initiator and TiCl4 are preferably about 10xe2x88x923 to 10xe2x88x921 molar (M) and 10xe2x88x923 to 10xe2x88x922M, respectively, based on the total solution, while the molar content of DTBP is adjusted to be equal to or greater than that of any protic impurity. After the above living polymerization is essentially complete, the comonomer according to formula (viii) is preferably added at a level of about 1.5 to 10 times that of the initiator and allowed to react for a period of about 1 to 2 hours.
In a second embodiment (random copolymerization mode), a monomer consisting essentially of isobutylene monomer, initiator (B) and styryl comonomer (C) are reacted together in the presence of a Lewis acid to provide the virtually telechelic polymer. In this case, the resulting polymer is less xe2x80x9ctelechelicxe2x80x9d in that functional silyl groups derived from comonomer (C) are less likely to reside as close to the tail as in the first embodiment. The proportions of the components, reaction temperatures and preferred values employed in this embodiment are essentially identical with those described in connection with the sequential mode, supra. However, in this case, components (A) through (C) are first mixed, preferably in solution, and the Lewis acid is then added to start the polymerization. This random copolymerization mode is preferred in view of its simplicity, even though it does not result in a product which is as close to telechelic as prepared by the sequential mode.
In the instant method, whether sequential or random, the halosilyl functionality of the product polymers is preferably converted to alkoxysilyl by reacting the polymer with an alcohol. During such an alcoholysis, the halogen groups on silicon are replaced by the corresponding alkoxy functionality. This reaction must be conducted under dry conditions so as not to prematurely hydrolyze either halogen or alkoxy groups. Typically, the reaction is carried out by adding the alcohol, which preferably has 1 to 4 carbon atoms, to the above described halosilyl-functional polymer solution at xe2x88x9280 to 60xc2x0 C., an excess of 500 to 2,000% of the alcohol over the stoichiometric amount being preferred. The final alkoxysily-functional polymer may then be isolated from the reaction solution by conventional means, such as precipitation with a non-solvent.
The reactive silyl-functional virtually telechelic polymers of the present invention can be cured by exposure to moisture and can therefore be employed as base polymers in the formulation of sealants, adhesives and coatings when combined with the appropriate filler, plasticizers and catalysts, inter alia. Those skilled in the art will, of course, appreciate that at least one crosslinker would be necessary to obtain a cured system when there is only one reactive group on silicon (e.g., when n=1 in formulas ix and x). When cured, these formulated compositions can be used as form-in-place gaskets, seals for insulating glass, and the like.